Add YAML selectors to manifest

[jtcohen6]

Describe the feature

Add a representation of YAML selectors to manifest.json, to support rendering in the docs site (dbt-labs/dbt-docs#119) and other downstream consumers.

How best to render YAML-defined selector logic into JSON? Here's @drewbanin in that issue:

• A json representation (not the raw YAML) would be ideal
• We probably do not want an entire list of nodes that would be matched by each selector, but would instead benefit from the "expanded" set of node selector description. It would be ideal if the node shorthand (eg models/my_dir/everything.*+) was expanded out into its full json object representation. This will make it easiest for the docs site (or other artifact consumers) to operate on IMO.

Describe alternatives you've considered

• Excluding YAML selector support from the docs site or downstream metadata consumers
• ???

Who will this benefit?

• Everyone who starts using YAML selectors in v0.18.0